In pressurized water nuclear reactors, the core generally consists of prismatic assemblies arranged vertically and side by side. The outer surface of the core has a complex shape possessing numerous steps.
Arranged around the core, inside the nuclear reactor vessel, there is a core containment consisting of a cylindrical shell with a vertical axis. Between the outer surface of the core and the inner surface of the core containment, there is an annular region of varying thickness. To hold the reactor core laterally and to channel the greater part of the cooling water flow into the core itself, a modular lining is arranged in this annular region, the said lining consisting of elements joined to one another, some of which can be fixed to the core containment.
This modular lining generally consists of metal plates screwed to one another, some of them being arranged horizontally over the whole width of the annular space, and the others being arranged vertically, in contact with the outer surface of the core.
The horizontal plates, called shape adaptors, are in contact via their outer edge with the inner surface of the core containment and via their inner edge with the outer surface of the core. These shape adaptors are arranged at different levels over the height of the core.
The shape adaptors can be fixed to the core containment by screws inserted in a horizontal bore passing through the core containment and bearing against the core containment with their heads, the shape adaptor possessing a bore in the extension of the bore of the core containment, which is partially tapped to enable screwing the threaded part of the screw.
The temperature of the reactor cooling water is higher inside the core than outside the core containment, because this water heats up in contact with the assemblies in which it circulates from bottom to top, and then, after cooling in the steam generators, returns to the base of the core through the peripheral region of the vessel around the core containment. The core containment is therefore in contact via its outer surface with cooling water at a temperature below the temperature of the water inside the core, and more particularly of the water in the upper part of the core.
The shape adaptors which are in contact both with the core containment and with the vertical plates forming the shield in contact with the outer surface of the core are therefore subjected to substantial thermal gradients. Furthermore, the shape adaptors which are subjected to the radiation emitted by the core are subjected to heating within their bulk. The same applies to the fixing screws. It is therefore necessary to avoid any confined space between the screws and their housing which could lead to vaporization of the water contained in this confined space and to heating and corrosion of the screw.
It is therefore necessary to cool the fixing screws of the shape adaptors and especially to make their temperature uniform.